This invention relates generally to techniques for scrambling and unscrambling television signals, and, more particularly, to improved scrambling and unscrambling techniques applied to a subscription television system controllable by a central computer.
In subscription television, or "pay-T.V." systems, subscribers select programs that they wish to view, and pay to have those programs transmitted to their television receivers, usually along a coaxial cable. One requirement for such systems is that the transmitted signals should be unintelligible to non-subscribers or to subscribers who have not paid for a particular program. Various methods have been suggested for scrambling video signals, such as by inserting time delays, or by inverting portions of the video signals so that white and black images are reversed on portions of the television screen.
The success of a particular scrambling technique depends, first of all, on whether a program is sufficiently scrambled to deter unauthorized viewers from watching it in a scrambled condition, and secondly, on how difficult it is for a resouceful viewer to circumvent the protection provided by the scrambling techniques.
Some prior systems provide for limited variation of the mode of scrambling and unscrambling, these generally requiring the insertion of a coded card, or the like, to correctly unscramble the signals. However, there has long existed a need for a scrambling technique in which security can be maximized by rapidly and automatically varying the scrambling mode, without the need for manual intervention by the subscribers, and which will effectively deter viewers of the scrambled television picture. The present invention fulfills this need.